The present invention relates to a system having one or more dynamoelectric machines driving a load. In particular, it relates to a control means for stopping a drive system having a large load driven by one or more synchronous motors.
It is common to drive large loads, such as grinding mills, by having a large diameter ring gear attached to the mill driven by pinions which are in turn mechanically coupled through clutches to one or two synchronous motors. Usually, driving power to the grinding mill is discontinued by open circuiting circuit breakers to cut power to the motors and by releasing the clutches. As a result, separately rotating pieces of equipment, i.e., motors and mill are left coasting. A synchronous motor may take as long as twenty minutes to run down to standstill. The mill normally begins to oscillate and may be in motion for three or four minutes. It is not possible to work on the mill until the mill has stopped oscillating; however, work normally does not begin on the mill until the motors have come also to a standstill so as to ensure the safety of the workers.
It is very desirable to have all moving units come to rest as quickly as possible to reduce the mill shutdown time as well as to reduce the risk of the load freezing in the mill.
One system that overcomes the above disadvantages is disclosed in U.S. Pat. No. 4,298,113 issued Nov. 3, 1981 to Shaver et al. In this patent, a wet clutch is utilized to mechanically couple the synchronous motors with the mill. Further, a brake is provided on the output side of the wet clutches. A distinction should be noted between wet clutches and dry clutches. Wet clutches transmit torque by fluid shear between the mating friction surfaces. The fluid, usually oil, is circulated in a very thin film between the clutch plates which are not in actual sliding contact. The heat is generated in the oil film between the plates and may be removed from the oil by external cooling means. Wet clutches have inherently high thermal capability, and as a result may be allowed to slip continuously without significant wear. Wet clutches are expensive, and tend to be mechanically critical. Conversely, dry clutches, where friction shoes actually rub against the mating clutch surfaces, have limited thermal capacity, cannot stand a significant amount of continuous slipping, are not mechanically critical, are a fraction of the cost of the wet clutch, and as such are widely used in industry. The aforementioned U.S. Patent discloses a method of stopping the mill comprising slowing the mill down until it stops rotating and thereafter slowly lowering the mill so that the load in the mill comes to rest at the bottom dead center position of the mill. To accomplish this, pressure in the wet clutches is reduced with the motors running at synchronous speed to permit the mill to slow until it stops rotating. Thereafter, the wet clutches provide sufficient torque to just balance the force created by the mill load which is held up at an angle. The angle is normally of a considerable size in the order of 55 to 60 degrees. This requires a large amount of heat dissipation because the motors are running at full speed and the torque required by the clutches to balance the load is large, perhaps in the order of 75 percent of full load torque. Clutch pressure is then gradually reduced to its minimum to permit the mill to approach bottom dead center, at which time the brakes are applied to hold the mill. The U.S. Patent further discloses that, if desired, the synchronous motors may then be stopped by disconnecting the power supply to the motors and engaging the clutches periodically to bring the synchronous motors to a stop. The problem with this method is that a significant amount of heat must be dissipated through the clutches. This amount being beyond the capability of dry clutches. Therefore, the type of clutch required to stop the mill quickly is limited to clutches that are able to dissipate large amounts of heat quickly such as wet clutches with external oil coolers. Wet clutches are considerably more expensive and add significant costs to the mill possibly making the purchase of the mill by a customer economically undesirable.